The present invention relates to tubular connections and more particularly to threaded tubular connections employed in joining lengths of pipe or joints of the kind commonly used in the oil and gas industry. In particular, the tubular connection of the present invention is a self-regulating, torque-resistant, threaded connection.
The trend in the oilfield is to minimize the diameter of pipe connections and to conserve borehole diameter. Two types of oilfield connections, namely integral flush joints and slim line high performance connections, have been utilized for these purposes. The outer diameter of a flush joint connection is substantially the same as the outside diameter of the body of the pipe. In other words, the connection is contained within the wall thickness of the pipe body. The outside diameter of conventional pipe couplings are typically 10 to 13% greater than the wall thickness of the pipe body. The outside diameter of a slim line high performance connection is generally 2 to 3.5% greater than that of the body of the pipe. Slim line high performance connections may be manufactured with or without cold swaged sections; hot-forged upset; and couplings. Though not undergoing as many make-up and break-out cycles as drill pipe, tubing connections must also remain serviceable after repeated makeups. So must casing connections to a lesser degree. An emerging technology, drilling with casing, requires casing connections with the added performance attributes of drill pipe tool joints.
U.S. Pat. Nos. 4,009,893; 4,538,840; 4,570,892; 4,611,838; 4,629,221; and 4,629,224 disclose various types of connections for tubular members having interfitting portions which serve to seal the connection. For example, U.S. Pat. No. 4,611,838 of Mannesmann discloses an annular end face of the pin member for opposing an annular shoulder of the box member, lying in planes transverse to the pipe axis. The pin member has an unthreaded annular bulge which engages an unthreaded frustoconical peripheral zone on the box member to form a seal.
A major deficiency in slim line and flush-type connections is their extremely low compression rating. Typically the flank angles of prior art threads are large, which results in large clearances between the compressive load bearing thread surfaces at full make up. Further as the flank angles are reduced, the clearances between the threads must be increased to permit the threads to stab into the grooves upon makeup. Thus, prior art connections provide a large clearance between the flanks of the threads. Large clearances between the threads permit movement between threads under cyclic loads and thus do not achieve a tight connection under cyclic loads. Large flank angles and thread clearances weaken the connection in compression. Prior art connections may have a 25 to 30% compression efficiency with a 60% tension efficiency. It also causes the connection to be weak in bending. Bending is compression on the inside of the connection and tension on the outside of the connection.
Square threads have substantially no flank angle and therefore are desirable because they provide good tension and compression load transfer. But in order for square threads to stab, the thread flank clearances must be so large that contact between the load flanks and stab flanks can not be achieved upon make-up. Thus, it is commonly believed that it is not possible to insert a square thread, including a tapered square thread, into an accommodating groove without having prohibitively large thread flank clearances.
Further it is commonly believed that for a thread to be "stabbable" and "machinable" a hook thread must have an included angle of 15.degree. or more, that a non-hook thread, such as API buttress thread, must have an included angle of 13.degree. or more and that a power thread such as a stub Acme thread or API X-line thread must have an included angle of 12.degree. or more. Square threads cannot be easily manufactured, particularly in small diameter connections. Therefore these prior art threads require these minimum included angles to be stabbable and be machinable. The prior art connections with modified square threads use variable width threads to permit the square threads to be stabbed.
The prior art wedge thread or dove-tailed thread was developed to increase thread contact and achieve locking threads. A wedge thread is a thread having load and stab flanks which have different helix angles, i.e. different leads. Since their pitch is cut off two leads, wedge threads have a variable pitch. Wedge threads obtain their wedging by monotonically increasing the thread within the groove as one member is rotated with respect to the other member. Wedge threads mate together by rotational and then axial movement. The wedging of a wedge thread occurs along the axial length of the threads with a larger thread width being received into a smaller root opening. Connections using wedge threads produce torsional resistance between the threads due to the different leads between the load and stab flanks, i.e. a dovetail, wedge-type thread. However, a wedge thread profile requires multiple machining passes to cut the thread.
The present invention overcomes the deficiencies of the prior art.